The present invention relates to a twin conical screw extruder for extruding a highly viscous material such as plastics, rubber or the like.
In the twin conical screw extruder, the container is generally reduced in section area in the direction toward the tips of the rotors. Accordingly, the internal pressure is increased in inverse proportion to such reduction. This is advantageous in that a strong extruding force is obtained for the rotor lengths.
The inventor of the present invention had proposed an extruder utilizing the advantage above-mentioned, which was allowed as U.S. Pat. No. 4,764,020.
In a conventional twin conical screw extruder including the prior art above-mentioned, the axes of bearings which support the base portions of two rotors, are naturally identical with the axes of the twin conical container which houses the rotors. Accordingly, the angle .alpha. formed by the axes of the bearings is equala to the angle .alpha. formed by the axes of the twin conical container. At a non-load state where the container is not filled with a material to be extruded, the rotor shafts are straight and do not interfere with each other even though the screw mountain portions are meshed with each other, so that gaps are formed between the rotors.
In the prior art, when the rotors are rotated with the screws meshed with each other, the internal pressure applied to the material becomes the highest in the vicinity of the points at which the screws are meshed with each other, i.e., the plane between and including the axes of the rotors. Further, the internal pressure is increased in the direction toward the tips of the rotors. Accordingly, the tips of the twin rotors are pushingly turned outwardly. Thus, at the time when a load is applied, the rotor shafts are so resiliently deformed as to be outwardly turned.
Taking such resilient deformation of the rotors into consideration, there have been determined that gaps exist between the container inner wall surface and the locus faces of rotation of the rotor mountain portions. Accordingly, such gaps are naturally made great. Further, since the resilient deformation of the rotors causes the gaps between the screw meshing points to be increased, the material filled in the gaps sticks to the rotors as they are rotated and is therefore not conducive to an extruding force. Thus, the performance of the extruder at the screw meshing portions is deteriorated. In this connection, there occurs a phenomenon that the increase in the pressure at the material extruding port is not proportional to the rotor drive force, but is weaken or saturated. This is more remarkable as the material viscosity is higher.
In the conventional twin conical screw extruder, each of the mountain portions of the screws of the rotors is made in the form of a trapezoid and the mountain portions of one screw are meshed, as fully embedded in, with the valley portions of the other screw.
With the screw arrangement above-mentioned, when there is extruded a highly viscous material of which frictional resistance is high at the interface with the screws, or when the rotor drive force is increased in order to increase the pushing force at the tip extruding port, the resistance force for pushing the material back from the rotor tips is also increased. Accordingly, the increment of the pushing force at the tip extruding port with respect to the increment of the rotor drive force is suddenly decreased and then saturated.